By use of a motor having a brush, electricity is supplied through the brush sliding on a commutator. Specifically, a coil wound around a core of a rotor is connected to the commutator, and when electricity is supplied to the coil, the rotor starts to rotate by virtue of the forces of attraction and repulsion applied from a permanent magnet provided in a housing so as to face the rotor.
In the motor having the above configuration, because the brush and the commutator is solid, when the motor is operated, the brush slides on the commutator depending on roughness of each surface of the brush and the commutator. Microscopically, the brush slides on the commutator with contacting to the commutator at three points, and these points are changed depending on each slide. Thus, by use of the motor having a brush, mechanical loss and electrical loss occur. Specifically, the mechanical loss includes such as wear on the brush and the commutator caused by the slide, and the electrical loss occurs when a contact voltage is reduced.
On the other hand, a known metal-graphite brush, which is applied to a vehicle, is made by mixing graphite particles and copper particles in a binder and sintering the mixture. (JP2001-298913A).
A known method for manufacturing such metal-graphite brush is as follows. First, natural graphite particles, as a base material, and a phenol resin solution, as a binder, are mixed. Next, a lubricant, such as molybdenum disulfide, is added to the mixture. Then, the mixture is sintered in a nitrogen-rich atmosphere at a temperature within a range of from 700 to 800° C. In this case, the film of dissolved phenol resin formed on the surface of the graphite particles is carbonized by a process of reduction sintering so as to become amorphous graphite. This amorphous graphite is used as a binder in order to bind graphite particles. In addition, because a part of the organic substances, originally included in the solution of phenol resin, sublimate as a carbon dioxide, or as water vapor, many porosities are formed on both the surface and the interior of the sintered material. By virtue of the hygroscopic property of the graphite particles, the metal-graphite brush, which is made of the graphite particles by the method described above, can absorb moisture in the atmosphere, as long as the metal-graphite brush is left in atmospheric air.
If the metal-graphite brush described above is attached to a motor, when the metal-graphite brush is operated, the temperatures of the slide surfaces of the metal-graphite brush and of the commutator rise. Accordingly, moisture, originally contained in inner porosities located near the slide surfaces of the metal-graphite brush, starts to vaporize. Then, the vaporized moisture gathers on the slide surfaces of the metal-graphite brush and the commutator. Therefore, because a coefficient of sliding friction between the slide surfaces of the metal-graphite brush and of the commutator is lowered by the vaporized moisture, in other words, because of the effects of gaseous lubrication, the degree of wears on the metal-graphite brush can be reduced.
However, when such motor having the metal-graphite brush is applied to the vehicle, because of heat in the engine room, temperatures at the slide surfaces of the metal-graphite brush and of the commutator may rise to 100 degree Celsius or more. In this case, because moisture, originally contained in inner porosities located near the slide surfaces of the metal-graphite brush, starts to vaporize rapidly, the vaporized moisture may not exist on the slide surfaces of the metal-graphite brush and the commutator. Thus, the metal-graphite brush slides on the commutator without the vaporized moisture on each of the slide surfaces, as a result, a coefficient of sliding friction between the slide surfaces of the metal-graphite brush and of the commutator is increased. Thus, when the motor having a metal-graphite brush is used at a high temperature, especially at 100 degree Celsius or more, the metal-graphite brush wears quickly, as a result, duration of life of the motor having a metal-graphite brush is shortened.
To avoid such problems, another process for making a metal-graphite brush is disclosed in, for example JP2004-173486A. In this process, porosities formed on the surface or inside of the sintered material of the metal-graphite brush are infiltrated with liquid having a boiling point higher than that of water. According to this invention, even when the motor is used under a temperature of 100 degree Celsius or more, moisture infiltrated inner porosities located near the slide surfaces of the metal-graphite brush does not completely vaporize, and the vaporized moisture exists on the slide surfaces of the metal-graphite brush and the commutator. Thus, a coefficient of sliding friction between the slide surfaces of the metal-graphite brush and of the commutator can be reduced, as a result, the degree of wear on the metal-graphite brush can be decreased.
Further electrical loss occurs when such metal-graphite brush is used in the motor. Specifically, the contact voltage of the metal-graphite brush is significantly reduced comparing to a contact voltage of a known metal brush. For example, when the metal-graphite brush including copper powder at 60% or more by weight and having relatively high current density is used, its contact resistance becomes 50 mΩ, and contact voltage between the brush and the commutator is reduced at from 0.4V to 0.5V.
To avoid such reduction of the contact voltage, another process for making a metal-graphite brush is disclosed in, for example JP05-236708A. Instead of copper powder, this a metal-graphite brush includes a conductive staple metal fiber, a compounding short fiber in which a conductive metal film is provided on a surface in a longitudinal direction of the carbon fiber; and a compounding short fiber, in which the conductive metal film is provided on a surface in a longitudinal direction of a conductive staple metal fiber and a surface in a longitudinal direction of the carbon fiber; in order to reduce the contact resistance of the brush.
As mentioned above, when the porosities of the metal-graphite brush are infiltrated with liquid having a boiling point higher than that of water, because of the effects of gaseous lubrication, even when the brush is used under a temperature of 100 degree Celsius or more, degree of mechanical wear of the brush can be reduced. However, in such metal-graphite brush, electrical loss has not been considered. For example, when the glycol type liquid or the glycol ether type liquid is used as liquid which infiltrates the porosities of the brush, because the glycol type liquid and the glycol ether type liquid has insulating property, electrical loss will be further enhanced.
Further, when the liquid having insulating property is applied to the slide surfaces, because electric resistance is enhanced as mentioned above, sparks may frequently occur on the surface of metal-graphite brush, as a result, degree of mechanical wear on the metal-graphite brush may be further increased.
On the other hand, when the metal-graphite brush uses the conductive staple metal fiber instead of copper powder, mechanical loss and electrical loss are not sufficiently improved comparing to another known metal graphite brushes.
Thus, a need exist to provide a metal-graphite brush or a motor having the metal-graphite brush by which mechanical loss and electrical loss can be preferably improved.